Valve-regulated, gas-recombinant, lead-acid batteries are well known in the art and comprise essentially a cell element having a plurality of positive and negative polarity plates stacked together so as to alternately interleave one polarity plate with the other and be separated one from the next by a fibrous, absorbent mat which capillarily immobilizes the electrolyte within the cell element while allowing oxygen to migrate therethrough.
It is also known to gang a plurality of high power such batteries together to form a battery pack for powering electric or hybrid-electric vehicles, and for reserve or standby power applications. In such applications, the batteries are packed closely together and air blown therebetween to remove the heat generated within the batteries over prolonged periods of usage. In this regard, lead-acid batteries are known to be particularly temperature sensitive in that exposure to elevated temperatures for prolonged periods of time is known to degrade the performance of the battery and shorten its useful life. Accordingly, it is desirable to remove the heat generated within the battery as quickly as possible, both during charging and discharging, with the goal being to have the battery's operating temperature be as close as possible to ambient temperature.
The ability to remove heat from the innards of gas-recombinant batteries is particularly difficult because the separator between the positive and negative plates of a gas-recombinant battery cell element absorbs and immobilizes the electrolyte, which, in turn, serves to help retain heat within the innards of the cell element. In this regard, flooded-electrolyte batteries which do not have electrolyte-absorbent mats have an inherent cooling mechanism built therein in that the electrolyte is free to circulate within the battery. Convective circulation incident to localized heating of the electrolyte carries the heat generated within the centermost regions of the cell element to the extremities thereof where it can be more readily dissipated through the container walls. Without the assistance of a mobile electrolyte, removal of heat from the centermost regions of the cell element of a gas-recombinant battery is dependent primarily on conduction through the materials comprising the cell element. The structure of the cell element, however, is such that the heat flows in directions parallel to the plates at a rate approximately four times greater than the rate heat flows in the direction perpendicular to the plates. This is due primarily to the fact that the lead grids supporting the active material of the positive and negative plates have a relatively high thermal conductivity and extend continuously to the periphery (i.e., top, bottom and sides) of the cell element. Heat flow in the direction perpendicular to the plates is impeded by the separator, the interplate gap and any over-pasted active material. Accordingly, in gas-recombinant batteries, the most effective region for extracting heat from within the cell element is around the periphery of the element. Unfortunately, compared to the ends of the cell element which have a relatively high surface area, the periphery of the element (i.e., where the plates emerge) presents a relatively small surface area from which to extract the heat. To compensate for this smaller peripheral area (i.e., so that one can effectively remove heat from within the cell element via the periphery thereof) it is important that the heat be removed at a rapid rate. It is imperative therefor that the thermal resistance between the cell element's periphery and the outside surface of the battery's container (i.e., where forced convective cooling is available) be minimized.
The thermal resistance between the periphery of the cell element and the outside surface of the battery container is made up of two principal components, (1) the sidewall of the container, and (2) the gap between the periphery of the cell element and the inside surface of the container sidewall. Both of these components typically comprise materials having relatively poor thermal conductivity, and hence their thermal resistance is significantly affected by their respective thickness. The thermal resistance resulting from too thick a sidewall can be reduced by making the sidewall as thin as possible. The gap between the cell element and the sidewall, however, is a particular problem in valve-regulated batteries which operate at superambient pressures (e.g., 1.5 psi or more). The internal pressure can cause sidewalls which are too thin/weak to bulge outwardly and, in so doing, increase the size of the gap between the periphery of the cell element and the inside surface of the container sidewall. Such an increased gap increases the temperature drop (.DELTA.t) across the gap, and negates any benefits that might otherwise be derived from reducing the thickness of the sidewall. This problem is exacerbated at superambient temperatures which tend to reduce the strength of the material comprising the sidewall and hence its ability to resist bulging.
It is an object of the present invention to provide a valve-regulated, deep-cycling, gas-recombinant, high energy lead-acid battery operating at superambient internal temperatures and pressures which is capable of effectively conductively dissipating heat generated within the cell elements through the sidewalls of the battery container with a minimum of thermal resistance contributed by the battery container sidewalls and internal spacing of the cell element from the container sidewalls. It is a further object of the present invention to enhance the ability of such sidewalls to dissipate such heat into a cooling medium flowing by the sides of the battery.
These and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.